1. Technical Field
The present invention relates generally to power conversion and in particular to power supplies. Still more particularly, the present invention relates to an over-voltage protection circuit for preventing system shutdown in a power system employing multiple power supplies and a method of operation thereof.
2. Description of the Related Art
Regulated DC power supplies are typically needed for most analog and digital electronic systems. Two major categories of regulated DC power are linear power supplies and switching power supplies. Generally, in linear power supplies, a transistor (operating in its active range) is connected in series with an input voltage source and the voltage drop across the transistor is automatically adjusted to maintain an output voltage at a desired level.
In switching power supplies, transformation of DC voltage from one level to another is accomplished typically by means of DC/DC converter circuits, such as step-down (buck) or step-up (boost) converter circuits. Solid-state devices, such as transistors, are operated as switches (either completely ON or completely OFF) within these switching converters. Since the power devices are not required to operate in their active region, this mode of operation results in lower power dissipation.
Furthermore, increasing switching speeds, higher voltage and current ratings of these power devices are some of the advantages that have increased the popularity of switching power supplies.
Switching power supplies that convert AC or DC input power into DC output(s) frequently have multiple outputs. These outputs are often derived from multiple secondary windings on a single power transformer. In a switching power supply, the primary winding of the a power transformer is switched or commutated to the input voltage source by power switches in such a way as to provide pulses at the appropriate current and voltage levels on the secondary outputs. The DC secondary outputs are formed via rectification and subsequent filtering of the pulse train on the transformer secondaries. Each DC output voltage level depends on a turns ratio of the respective secondary windings to the primary winding as well as the ratio of the pulse width to the switching period.
The DC output voltages are then directly or indirectly regulated by a control feedback circuit. Direct regulation occurs when the feedback circuit senses at least one of the DC outputs (usually called the main output) and then modifies the switching pattern of the power switches to compensate for the changes in the load or in the input voltage, thereby keeping the DC voltage level on the regulated main output relatively constant. The are many possible methods for switching power supply regulation including, for instance, pulse width modulation (PWM). PWM, as a matter of fact, is one of the more widely used control and switching method.
There is a growing demand, e.g., in the telecommunication and computer industries for increased current handling capabilities in the power supplies employed to provide regulated power to their equipment. A common approach to increase the current handling capacity of existing DC/DC power supplies is to add additional power stages to an already existing power supply. This distributive approach provides for greater expandability of a power system, permitting the utilization of lower current distribution buses. Furthermore, a distributive power system with the ability to use standard power supplies allows for redundancy, which in turn, increases the reliability of the entire power system. Examples of distributed power systems include the use of power supplies with their own individual input filters or different input buses. Other distributed systems may employ power supplies that share a common output bus, i.e., parallel outputs, for increased power or redundancy.
In a paralleled distributed redundant power system, i.e., the power supplies share a common output bus, an over-voltage condition is typically sensed before an isolation device, such as a diode, that is employed in each output terminal to provide electrical isolation between the paralleled outputs. This is to prevent shutdown of the entire power system in the event of an over-voltage fault on any one of the outputs of the paralleled power supplies. For example, if a +5V output common bus experiences an over-voltage condition due to a failure in one of the paralleled power supplies, the remaining "good" power supplies will not shutdown because of the over-voltage sense point being on the anode side of the isolation diode. A good power supply should never sense the over-voltage fault as its isolation diode becomes reverse biased for an over-voltage condition on the common bus.
Distributed power systems, however, have inherent shortcomings. Whenever anyone of the outputs is in an over-voltage condition, the anode voltage of an isolation diode in a good power supply becomes lower than its corresponding cathode voltage. Once its isolation diode becomes reverse biased, the power supply cannot conduct power to the common bus to power a load. Since the power supply regulation sense point is located at the cathode of its isolation diode, the power supply is forced to operate in a skip or discontinuous mode. The situation is further exacerbated if the output that suffered an over-voltage condition happens to be the "master" output for a multiple output power supply. Once the power supply under fault is powered down, the common bus voltage begins to decay until the regulation control loop responds. Under medium to full load condition, the output voltage may drop to below a regulation point, resulting in the loss of a "power good" signal, which in turn, will initiate shutdown of the entire system.
Accordingly, what is needed in the art is an improved power supply that mitigates the above mentioned limitations. In particularly, there is a need in the art for an over-voltage protection circuit for preventing system shutdown in a power system employing multiple power supplies.